This invention relates to vehicle power systems and, more particularly, to solid state power controls.
Vehicles, such as aircraft, typically utilize one or more power distribution units to distribute power from a primary power source to various vehicle systems. The solid state power controls in a power distribution unit typically include an electronic switch, such as a FET, and electronic circuitry that provides wiring protection. The FET and circuitry are often referred to as a solid state power controller (“SSPC”). The SSPC has found widespread use because of its desirable status capability, reliability, and packaging density. A typical power distribution unit may include hundreds or thousands of SSPCs.
SSPCs also must operate in the presence of lightning, which can adversely impact electronic devices. Traditionally, aircraft had an aluminum skin that attenuated the lightning current induced on the wires. Some aircraft now use composite materials instead of aluminum for weight and strength benefits. However, composite materials do not provide the same level of attenuation to lightning as aluminum. When lightning occurs, hundreds of volts may surge between a load in the vehicle system and the aircraft chassis. As such, the lightning requirements of SSPCs have increased.
The increase in lightning levels poses a significant additional burden because the SSPC does not provide galvanic isolation in the off state, as would a typical electro-mechanical circuit for example. Instead, the SSPC uses the FET for switching and the electronic circuit to provide the circuit breaker function. If the SSPC is in an off state when lightning strikes, the large voltage potential will undesirably increase the voltage across the FET. An over-voltage clamp can be used to protect the FET from exceeding its maximum voltage capability by placing the FET into a linear region. However, the increased power dissipation of the FET in the linear region limits the amount of lightning energy that can be dissipated. Alternatively, high voltage FETs may be used to block the voltage in the off state, transient suppression devices can be placed across the FETs or more parallel FETs may be added, but these solutions are expensive, require larger packaging, and reduce reliability. Additionally, to protect the loads, a transient voltage suppression is often used by the load to shunt or divert the lightning current. This further increases the lightning energy that the SSPC must survive.
There is a need for a simple, relatively inexpensive SSPC with improved lightning protection. This invention addresses those needs while avoiding the shortcomings and drawbacks of the prior art.